1. Field of the Invention
This invention relates to the non-destructive measurement of parameters of semi-conductor material and, more particularly, to the flaw testing of semi-conductors by measuring local carrier concentration.
2. Prior Art
The semiconductor fabrication process must carefully control parameters of the material (such as carrier concentration, electron and hole mobility, impurity content, built-in strain, heterojunction abruptness, alloy composition, and film thickness) to produce reliable devices at reasonable costs. As device yield depends upon the uniformity of these parameters over a wafer, if tests can measure these parameters over the entire wafer, bad wafers can be discarded early in the production process. This avoids processing defective wafers and, thus, allows a more cost-effective production of semiconductor devices.
Standard methods now used to evaluate wafers include Van der Paaw and polaron capacitance, microscopic, photoluminescence, secondary ion mass spectroscopic, and transmission electron microscopy. However, these methods only analyze a small section of the wafer and the methods destroy the wafer if they test more than a small section. Also, standard methods do not find end homogenities in carrier concentrations caused by imperfect crystals made early in the chip production process. While non-destructive photoreflectance techniques help overcome these drawbacks, conventional focusing means limit the areas that can be tested.
U.S. Pat. No. 4,953,983, Bottka et al, entitled Apparatus and Method for Non-Destructively Measuring Local Carrier Concentration and Gap Energy in a Semi-Conductor, discloses a monochromatic method for using photoreflectance and the Franz-Keldysh relationship to measure semi-conductor properties. The Journal of Electronic Materials, also discussed that method and apparatus in Vol. 17, page 161, 1988. In the apparatus and method disclosed in these publications, the monochrometer's aperture slit spreads the single wavelength probe over a relatively large area on the sample.